---
title: "Time division switching and space switch"
subject: "Telecommunications"
chapter: 4
key: "semester_8/telecommunications:time-division-space-switch"
source: https://www.bctengineer.com/q/semester_8/telecommunications/time-division-space-switch
site: BCT Engineering
---

# Time division switching and space switch

Telecommunications · Chapter 4: Digital Switching

- Asked in: 2071 Shrawan, 2+5 marks
- Video: [Telephone Switching||Time Division Switching||Space Division Switching||Switching](https://www.youtube.com/watch?v=c775sTZOewg) — Engineering Basics
- Answer checked by Claude Opus 5.5 (high effort) on 2026-10-14
- Page: https://www.bctengineer.com/q/semester_8/telecommunications/time-division-space-switch
- Whole chapter: https://www.bctengineer.com/md/semester_8/telecommunications/chapter-4

## Question

What is the principle of time division switching? Describe the operation of time division space switch.

## Short answer

Time division switching shares hardware by assigning each call a time slot in a 125 µs frame. A time-multiplexed space switch uses an N×M crosspoint array and a control memory per output column; each memory word closes a crosspoint in its slot, connecting an input sample to the output while retaining the time slot.

## Answer

#### Principle of time division switching
In **time division switching**, the switching hardware is shared in time: each call is given a short **time slot** in a repeating frame and the hardware connects that call only during its slot. In PCM telephony the voice is sampled every 125 µs (8 kHz) and coded into 8 bits; 32 slots form one 125 µs frame. A crosspoint or memory location used by one call in slot 3 can be used by another call in slot 4. Two forms exist:
- **Time division space switching:** a crosspoint matrix is shared in time (changes highway, keeps slot).
- **Time division time switching:** samples are stored and read in a different order (changes slot, keeps highway).

#### Time division space switch (TMS)
A **time-multiplexed space switch** connects $N$ input TDM highways to $M$ output highways. It has an $N \times M$ crosspoint matrix (electronic gates or multiplexers) and a **control memory** for each output column (output-controlled) or each input row (input-controlled).

```text
            O1      O2     ...   OM
 I1 ------- x ----- x ---------- x
 I2 ------- x ----- x ---------- x
 ...
 IN ------- x ----- x ---------- x
            |       |            |
         [CM 1]  [CM 2]   ...  [CM M]
     c words x log2(N) bits each
             ^
      time-slot counter (0..c-1)
```

#### Operation
1. Each column control memory has $c$ words, one per time slot. Word $t$ holds the number of the input highway to connect to that output in slot $t$.
2. A time-slot counter steps through the slots. In slot $t$ it reads word $t$ of every control memory.
3. A decoder for each column closes the one crosspoint named by the word, so the sample on that input highway passes to that output highway during slot $t$.
4. In the next slot new words are read and different crosspoints close. So each crosspoint is shared by up to $c$ calls per frame.
5. The processor writes the control memories during call set-up and clears them at release.

**Example:** in slot 5, CM2 holds "3", so crosspoint (I3, O2) closes and the sample in slot 5 of I3 goes to slot 5 of O2. In slot 6, CM2 may hold "1", connecting I1 to O2.

#### Features
- The sample keeps its **time slot**; only the highway changes. To change slots, a time stage is added (TS, ST, TST).
- Control memory size: $M$ memories of $c \times \lceil\log_2 N\rceil$ bits.
- The crosspoint count is $N \times M$, but each crosspoint serves up to $c$ calls, so the capacity is very large.
- Speed of the gates limits the number of slots per frame.
- On its own it cannot move a call to a different slot, so it is used as the middle stage of TST switches.

## Check yourself

The answer's key points as multiple-choice questions, with the correct option and why.

1. What is the principle of time division switching?
   A. Each call has its own dedicated crosspoint
   B. Hardware is shared in time by giving each call a slot in a repeating frame
   C. Calls are switched by frequency
   D. Calls are stored in relays
   Answer: B. A crosspoint or memory used by one call in one slot serves another call in the next slot. (Key point: Principle)

2. What does word t of a column's control memory hold?
   A. The output number
   B. The input highway to connect to that output in slot t
   C. The time slot to read
   D. The sample value
   Answer: B. The word names the input highway for that output in slot t. (Key point: Control memory content)

3. What does a time division space switch change?
   A. Highway only; slot is kept
   B. Slot only; highway is kept
   C. Both
   D. Neither
   Answer: A. The sample keeps its time slot; a time stage is needed to change it. (Key point: Slot behaviour)

4. Size of each column control memory with c slots and N inputs?
   A. N x c bits
   B. c words of ceil(log2 N) bits
   C. log2 c words of N bits
   D. c x c bits
   Answer: B. There is one word per slot, each identifying one of N inputs in log2 N bits. (Key point: Control memory size)

## Related in Telecommunications

- [Time and space switch drawbacks](https://www.bctengineer.com/question.md?key=semester_8%2Ftelecommunications%3Atime-space-switch-drawbacks) — Telecommunications, chapter 4
- [Space switch and why TST is used](https://www.bctengineer.com/question.md?key=semester_8%2Ftelecommunications%3Aspace-switch-why-tst) — Telecommunications, chapter 4
- [Space switch and two-stage S-T drawbacks](https://www.bctengineer.com/question.md?key=semester_8%2Ftelecommunications%3Aspace-switch-two-stage-st) — Telecommunications, chapter 4
- [Time switch and two-stage T-S network](https://www.bctengineer.com/question.md?key=semester_8%2Ftelecommunications%3Atime-switch-two-stage-ts) — Telecommunications, chapter 4
- [Space and time switches; STS vs TST](https://www.bctengineer.com/question.md?key=semester_8%2Ftelecommunications%3Aspace-time-switch-sts-tst) — Telecommunications, chapter 4
- [Space switch design with connection memory](https://www.bctengineer.com/question.md?key=semester_8%2Ftelecommunications%3Aspace-switch-connection-memory) — Telecommunications, chapter 4

## Related in other subjects

- [Time division multiplexing explained](https://www.bctengineer.com/question.md?key=semester_5%2Fdata_communication%3Atdm-explain) — Data Communication, chapter 6, asked 3×
- [Division operation and hardware](https://www.bctengineer.com/question.md?key=semester_5%2Fcomputer_organization_and_architecture%3Adivision-hardware) — Computer Organization and Architecture, chapter 5
- [CMOS switch operation](https://www.bctengineer.com/question.md?key=semester_2%2Fbasic_electronics_engineering%3Acmos-switch) — Basic Electronics Engineering, chapter 3
- [Four-segment instruction pipeline and space-time diagram](https://www.bctengineer.com/question.md?key=semester_5%2Fcomputer_organization_and_architecture%3Afour-segment-pipeline) — Computer Organization and Architecture, chapter 4, asked 5×